ORIGINAL PAPER

The Tribofilm Formation of ZDDP Under Reciprocating PureSliding Conditions

Yasunori Shimizu1,2 • Hugh A. Spikes1

Received: 12 August 2016 / Accepted: 17 October 2016 / Published online: 2 November 2016

� The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract The anti-wear and anti-seizure performance and

action mechanisms of zinc dithiophosphate (ZDDP) have

been investigated under reciprocating pure sliding condi-

tions to simulate piston ring and cylinder liner assembly,

using new techniques. The Mini Traction Machine–Space

Layer Imaging is a useful method for monitoring tribofilm

formation by ZDDPs. However, tests are generally carried

out in mixed sliding–rolling conditions and ZDDP film

formation in reciprocating pure sliding conditions is rarely

investigated. In this paper, the authors describe an inves-

tigation of ZDDP film formation in stationary ball on

reciprocating disc pure sliding conditions and compare the

results to those obtained in unidirectional pure sliding

conditions. In unidirectional pure sliding conditions, the

worn area on the ball expands with test time. By contrast,

in reciprocating pure sliding conditions, tribofilm forms on

the stationary ball and no significant damage occurs. In the

initial tribofilm formation under reciprocating pure sliding

conditions, solid particulate tribofilm with a high concen-

tration of S forms initially in the contact area and subse-

quently breaks up. During further rubbing, a Zn- and P-rich

tribofilm forms on the comminuted sulphur-rich tribofilm

and also the area where the initial tribofilm was removed.

Keywords Zinc dithiophosphate � ZDDP � ZnDTP �Tribofilm � Wear � Anti-wear � Pure sliding �Reciprocating � MTM � SLIM

1 Introduction

As a consequence of the need to reduce the fuel con-

sumption of passenger vehicles, the compression ratio and

the specific power of internal combustion engines have

been steadily increasing throughout the last 20 years [1, 2].

Because of this, cylinder liner temperature near the com-

bustion chamber, especially at top dead centre, has

increased so the conditions have become more severe for

wear and scuffing behaviour of piston ring cylinder liner

contacts [3]. In these more severe conditions, engine oil

formulations are required to maintain good anti-wear and

anti-seizure performance. Since zinc dialkyldithiophos-

phates (ZDDPs) are widely used in engine oils, primarily as

anti-wear additives, many researchers have studied their

anti-wear and anti-seizure performance under reciprocating

pure sliding conditions which simulate piston ring and

cylinder liner assembly. In previous research, friction

machines such as the Optimol SRV and Cameron Print

reciprocating rig have been employed and tribofilms

formed on the specimens after rubbing tests have been

investigated [3–9]. Recently, a new approach to monitor

ZDDP film formation process under pure sliding conditions

using AFM (atomic force microscopy) was introduced by

Gosvami et al. [10]. This method can track anti-wear film

formation in situ at the nanometre level.

The Mini Traction Machine–Space Layer Imaging

(MTM–SLIM, PCS Instrument) is also a well-known and

useful method to monitor ZDDP tribofilm growth in situ, and

its use to study the kinetics of ZDDP tribofilm growth has

& Yasunori Shimizu

yasunori.shimizu@idemitsu.com

1 Tribology Group, Department of Mechanical Engineering,

Imperial College London, South Kensington, Exhibition

Road, London SW7 2AZ, UK

2 Lubricants Research Laboratory, Idemitsu Kosan Co., Ltd,

24-4, Anesakikaigan, Ichihara-Shi, Chiba 299-0107, Japan

123

Tribol Lett (2016) 64:46

DOI 10.1007/s11249-016-0776-6

been widely reported [11–21]. In comparison with the new

AFM technique, the MTM–SLIM method can observe a

larger area and employ more flexible test conditions. In the

past, MTM–SLIM has been generally carried out in mixed

sliding–rolling conditions [11–20] and, very recently, in uni-

directional pure sliding conditions [21]. ZDDP film formation

in reciprocating pure sliding conditions has not yet been

investigated using the MTM–SLIM method even though the

behaviour of specimens in reciprocating conditions is likely to

be different from that in unidirectional rolling conditions

since mean speed and acceleration are continuously changing

in reciprocating conditions, while these parameters are con-

stant in unidirectional rolling conditions.

In this paper, ZDDP tribofilm formation is therefore

monitored in reciprocating pure sliding conditions using

MTM–SLIM.

2 Test Methods

The experiments in this work are conducted in a MTM–

SLIM ball on disc test rig as shown in Fig. 1a. In the normal

operation of this rig, a 19.05-mm-diameter ball made of

AISI 52100 (Ra = 0.02 lm) is loaded against a flat sur-

face of a 46-mm-diameter steel disc (AISI 52100, Ra =

0.01 lm) which is immersed in the oil sample. The ball and

the disc are continuously driven by separate electric motors.

At set intervals, motion is halted and the stationary ball is

raised and loaded upward against a coated glass window

(Fig. 1b). An interference image of the contact between the

ball and the glass window is captured from a camera in order

to record tribofilm formation on the ball.

When mixed sliding–rolling conditions are used and the

ball rotates, a tribofilm forms all around the ball (and the

disc). This means that when the ball is loaded against the

SLIM window, part of the rubbed track is always pressed

against this window (Fig. 2a). However, in a recent paper

the authors describe a study of ZDDP film formation in

pure sliding conditions with the disc rotating but the ball

stationary [21]. In this case, tribofilm forms on the ball only

at its point of contact with the disc because the ball is fixed

(Fig. 2b). Therefore, in order to measure the tribofilm

formed on the ball a procedure was developed in which the

ball shaft was rotated by 180� prior to image capture, as

shown schematically in Fig. 2.

In this previous study, the disc was rotated continuously

to give unidirectional pure sliding and this was compared

with behaviour in mixed sliding–rolling [21]. The current

paper describes the behaviour of ZDDP in reciprocating

pure sliding. In this, the ball is held stationary, while the

disc is reciprocated with a stroke length of 4 mm. This

behaviour is compared with ZDDP behaviour in unidirec-

tional pure sliding and in mixed sliding–rolling. Table 1

shows the main test condition used. For most work, the

load, disc frequency, stroke length, mean speed and the

lubricant temperature were kept constant at 20 N (maxi-

mum Hertzian pressure Pmax = 0.82 GPa), 20 Hz, 4 mm,

160 mm/s and 130 �C, respectively. However, the influ-

ence of load, disc frequency, stroke length and temperature

were also investigated.

3 Test Lubricant

All lubricant test samples were solutions of a ZDDP in

polyalphaolefin base oil at a concentration that corre-

sponded to 0.08 wt% of phosphorus. No other additives

Fig. 1 a Schematic image of MTM–SLIM during a friction test.

b Schematic image of MTM–SLIM while capturing an image

Fig. 2 Difference in the test methods between mixed sliding–rolling

and pure sliding conditions [21]

46 Page 2 of 11 Tribol Lett (2016) 64:46

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were present. The ZDDP employed was a C8 primary type,

and the base oil had a kinetic viscosity of 4.1 mm2/s at

100 �C, 2.6 mm2/s at 130 �C and a viscosity index of 124.

4 Test Results

4.1 Influence of Disc Movement

In pure sliding reciprocating conditions, the disc is recip-

rocating and the ball is stationary. Therefore, tribofilm

forms only on a point area on the ball as shown in the

SLIM image in Fig. 3. In this and subsequent images, the

reciprocating direction is horizontal (Fig. 3). In this figure,

the diameter of the rubbing contact region on the ball (ca

216 lm) considerably smaller than the diameter of the

Hertzian contact formed between the steel ball and the

glass window (ca 272 lm) so the rubbed region lies fully

within the SLIM image.

Figure 4 compares the results for tests performed under

unidirectional and pure sliding reciprocating conditions.

The load and temperature are listed in Table 1, while in

unidirectional sliding the continuous disc speed was

160 mm/s and thus the same as the mean reciprocating disc

speed. It can be seen that the contact behaviours are very

different in the two types of rubbing. In continuous sliding,

the contact region grows rapidly, indicating a large amount

of wear (although, as shown in Fig. 13 [21], some ZDDP

tribofilm still forms). By contrast, in reciprocating condi-

tions the rubbed area remains almost constant indicating no

significant wear of the ball. After 1 min of sliding, initial

tribofilm forms near the edges of the contact, but less tri-

bofilm forms in the central region. Later in the test, tri-

bofilm develops to cover the whole rubbing area.

4.2 Effect of Load

Pure sliding reciprocating tests were carried at the conditions

listed in Table 1 but at four different applied loads (5, 10, 15

and 20 N, corresponding to Pmax = 0.52, 0.65, 0.75 and

0.82 GPa). Figure 5 shows the resultant SLIM images of the

ball contact. Tribofilms start forming within 30 s from the

beginning of the test over a set area that depends on the load.

The film then thickens to cover this area. There appears to be

no significant influence of load on the tribofilm forming

process, and no surface damage was observed on the balls

under any of these conditions. The areas of tribofilms were

different at various load conditions because the theoretically

calculated Hertz diameter areas were different. However, in

each load condition, the diameter of tribofilms was close to

the theoretically calculated Hertz diameter (134, 170, 194

and 216 lm for the four loads, respectively). For the results

at 60 min under 20 N, the diameter of worn area was

approximately 0.22 mm which corresponds closely to the

theoretically calculated Hertz diameter.

For the results after 60 min from the beginning of the

tests, the tribofilm regions have elliptical shape, being

slightly longer in the reciprocating direction than trans-

verse to this. One possible reason for this may be that the

tribofilms are swept to the front and rear of the recipro-

cating directions and accumulate in these locations.

4.3 Effect of Test Duration

In the results in Fig. 5, it can be seen that separate tri-

bofilms are formed initially near the front and rear of the

contact area. In order to investigate this initial tribofilm

formation further, SLIM images were captured every 5 s

until 100 s for two repeat tests at 20 N load (Fig. 6). The

speeds of tribofilm growth are somewhat different, but both

tests show the same tribofilm formation process. In the first

5 s, an initial tribofilm is created in the centre of the con-

tact, having elliptical shape oriented with the long axis

transverse to the sliding direction. After 10-s rubbing, the

tribofilm patch becomes practically circular. On further

rubbing, this tribofilm then separates into two regions near

the front and rear of the contact, before eventually

reforming from the top and bottom between these two

regions to form a complete film.

4.4 Effect of Speed

Figure 7 shows the results for reciprocating tests performed

at 5, 10, 15 and 20 Hz and thus at different mean rubbing

speeds (Table 2). The rate of tribofilm formation was

slower when the frequency was decreased, but the overall

process of tribofilm growth was the same. After 30 s at 5

and 10 Hz, initial tribofilms were created in the centre of

Table 1 Standard test condition under pure sliding reciprocating

condition

Load (N) 20 (Pmax = 0.82 GPa)

Ball speed (mm/s) 0

Disc frequency (Hz) 20

Stroke length (mm) 4

Average disc speed (mm/s) 160

Temperature (�C) Always 130

Fig. 3 Reciprocating direction

in SLIM images

Tribol Lett (2016) 64:46 Page 3 of 11 46

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the contact area. After 1 min at 5 and 10 Hz, but after 30 s

at 15 and 20 Hz, two tribofilms formed near the edges of

the contact area. These film regions joined together after a

time that decreased with increasing reciprocating fre-

quency. After 60 min, the tribofilms were continuous and

elliptical at all frequencies.

4.5 Effect of Stroke Length

Figure 8 shows the results of tests performed using 4, 8 and

16 mm stroke lengths, all at 10 Hz, as well as one 4-mm-

stroke-length test performed at 20 Hz (Table 3). The latter

has the same average speed as the 8-mm/10-Hz test. For

the 4-mm-stroke-length/10-Hz test, the sizes of the two

regions near the edges at 1 min were similar to the ones

obtained at 30 s at 4-mm/20-Hz conditions. For the tests

carried out using 8 and 16 mm stroke lengths, the sizes of

the edge regions were smaller than the ones obtained

employing a 4 mm stroke length. In addition, there was no

significant influence of stroke length on the rate of tribofilm

formation although the sliding distance was increased by

changing the length. One explanation may be that the

Fig. 4 Comparison of SLIM

images of the wear scar on the

ball under different disc

movements

Fig. 5 Series of SLIM images of the ball in pure sliding reciprocating at various loads

Fig. 6 Series of SLIM images of the ball at the initial stage

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lubrication regime shifted to higher lambda ratio condi-

tions, less effective for building up ZDDP tribofilm, as the

average and maximum speeds increased. However,

although the sliding speeds at 10 Hz and 8 mm stroke are

the same as at 20 Hz and 4 mm stroke, the tribofilm for-

mation showed different processes under these two sets of

conditions. This suggests that not only the speed but also

stroke length affects tribofilm formation.

4.6 SEM–EDX Results

Scanning electric microscope–energy-dispersive X-ray

spectroscopy (SEM–EDX) was used to study some of the

tribofilms formed on the ball in pure sliding reciprocating

tests. A SEM, HITACHI S-3400N, was used to capture

high-resolution images of tribofilm surface topography,

and EDX, an Oxford X-ray System INCA, was used to

Fig. 7 Series of SLIM images of the ball at various disc frequencies

Table 2 Test conditions for disc frequency effect

Load (N) 20 (Pmax = 0.82 GPa)

SRR (%) 200

Ball speed (mm/s) 0

Disc frequency (Hz) 5 10 15 20

Stroke length (mm) 4

Average disc speed (mm/s) 40 80 120 160

Temperature (�C) Always 130

Fig. 8 Series of SLIM images of the ball at various stroke lengths

Table 3 Test conditions for stroke length effect

Load (N) 20 (Pmax = 0.82 GPa)

SRR (%) 200

Ball speed (mm/s) 0

Disc frequency (Hz) 10 20

Stroke length (mm) 4 8 16 4

Average disc speed (mm/s) 80 160 320 160

Temperature (�C) Always 130

Tribol Lett (2016) 64:46 Page 5 of 11 46

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analyse the chemical properties of tribofilms. In this study,

the working distance was 10 mm ± 1 mm and the accel-

erating voltage was 10 or 15 keV; from these voltages, it

can be estimated that the beam enables the detection of

chemical information about 1 micron depth into the sam-

ples. Six test specimens were prepared under different test

conditions. All rubbing tests except image (d) were carried

out at the conditions listed in Table 1 but at different test

duration in order to obtain the samples at the different

stages of tribofilm formation. The image (d) was tested

under 4 mm stroke length/10 Hz. For the images (a) and

(b) in Fig. 9, the test was stopped after 5 s and 20 s, before

the initial film separated into front and rear film regions.

For the images (c) and (d), the test was halted after this

separation (30, 60 s under 10 Hz). For (e) and (f) images,

tests were stopped after the separate regions of the tribofilm

in the front and rear started to join together (60 s, 30 min).

From the SEM images, individual solid particles can be

seen clearly in the images (a)–(d), which were captured

before the tribofilm reformation started. In images (e) and

(f), a pasty substance is observed. From the SEM–EDX

analysis in Fig. 10, it can be seen that for the first four

images S intensity is stronger than those of P and Zn.

However, when analysing the remaining two images, it is

observed that the ratio of intensities changes and the Zn

shows the strongest intensity.

Supporting evidence of these changes of intensities was

obtained using the EDX mapping technique (Fig. 11). In

the first 4 images, when the tests were stopped before the

tribofilm reformed, the S intensity was stronger than the

others and S, P and Zn were observed only where there

were solid particles. After the film started to reform, the Zn

signal was strongest and the darker area in the SEM image

showed Zn most strongly.

From these results, it can be concluded that two different

types of tribofilm are created, displaying different proper-

ties depending on whether they are examined before and

after the occurrence of film reformation. In other word,

S-rich tribofilm is mainly created on steel ball surface, and

then, P- and Zn-rich tribofilms replace and/or cover the

S-rich tribofilm.

4.7 AFM Results

In order to study the tribofilm thickness of ZDDP films,

AFM was used to analyse the MTM ball surface over the

same area as the chemical analysis using SEM–EDX. In

this study, the apparatus was a WITec alpha 300A and the

cantilever was a WITec AFM Arrow Cantilever Reflex-

Coated. Results are shown in Fig. 12. At 5 s after the test

started at 130 �C, the tribofilm regions were up to 700 nm

in height and 16 lm in width. Before tribofilm separated

(20 s), its height did not change significantly; however, the

tribofilm area expanded to 60 lm. Shortly before the sep-

aration into front and rear regions (30 s), the tribofilm

particles broke up into smaller sized ones, 350 nm in height

and 10 lm in width. After the onset of reformation, the

height of tribofilm reached 130 nm.

Since the initial particles before the separation showed

strong S intensity and also the reformed tribofilms showed

strong Zn and P intensity in Fig. 10, it can be concluded

that the particles are sulphur rich, while the final film

formed is ZDDP tribofilm. This sequence of sulphide fol-

lowed by phosphate is in accord with the HSAB principle

[21, 22]; however, the morphological changes are charac-

teristic of pure sliding reciprocating conditions; the par-

ticulate tribofilm grows very rapidly but then breaks up and

is swept to the front and rear of the reciprocating

Fig. 9 SEM images obtained under various test conditions

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directions. During further rubbing, a Zn- and P-rich tri-

bofilm forms on the comminuted sulphur-rich tribofilm and

also the area where the initial tribofilm was removed.

4.8 Comparison with Results in Unidirectional Pure

Sliding Conditions

It is of interest to compare the behaviour seen in recipro-

cating sliding with that seen and reported previously in

unidirectional sliding conditions [23]. As shown in Fig. 4,

in unidirectional conditions, the worn area on the stationary

ball surface expands with time, whereas no significant

damage is observed in pure sliding reciprocating condi-

tions. From the previous research by the authors (Figs. 13,

14) in [23], in the unidirectional pure sliding conditions

shown in Fig. 4, after 30 s from the beginning of the test,

550-nm-thick particles were observed on the ball with

strong S intensity in EDX analysis, but after further rub-

bing, a film with a high concentration of Zn and P was

formed. The sequence of tribofilm formation thus shows

similarities between unidirectional and reciprocating con-

ditions, even though wear is observed only in unidirec-

tional pure sliding conditions.

It is not clear why the wear behaviour is so different in

unidirectional and reciprocating conditions, but this may

relate to whether and to what extent a tribofilm forms on

the moving disc surface, which may depend on the pro-

portion of time that any part of the disc is in contact. In

explore this, tribofilms on the disc were also analysed using

SEM–EDX. Five discs were prepared under different test

conditions: two discs after 30- and 60-s unidirectional tests

with the same test conditions as those in Fig. 13, two discs

from reciprocating tests after 5 and 30 s as shown in Fig. 9

and the last disc from a 10-Hz, 16-mm-stroke-length 120-s

test as shown in Fig. 8.

From SEM results at 30 s in unidirectional conditions

and 5 and 30 s in reciprocating condition, unlike on the

ball, no particles were found on the disc (Fig. 15). There-

fore, it can be said that the very thick sulphur-rich partic-

ulate tribofilm forms only on the stationary surface. For the

EDX results, the intensities of Z, P and S were compared to

that of carbon in the disc material AISI 52100. After 30 s

in unidirectional pure sliding, Zn, P and S were observed,

but after 60 s, the intensity of all three of these peaks

diminished. This indicates that in unidirectional sliding,

some tribofilm forms on the disc surface initially, but this is

removed as further wear occurs. By contrast, in recipro-

cating tests after 5 and 30 s the intensities of Zn, P and S

became stronger with rubbing time although the initial

sulphur-rich tribofilms on the ball were removed. The film

present after 120 s at 16 mm/10 Hz was similar, and

comparison of the 4- and 16-mm tests indicated no sig-

nificant influence of stroke length on tribofilm on the disc.

Since the sliding length per one cycle in unidirectional

sliding is about 132 mm, this length is still much longer

than the 16 mm stroke length and it is possible that this

limits the ability of a tribofilm to form on the disc surface.

As a final confirmation of the importance of a tribofilm

forming on the disc surface, one additional unidirectional

test was carried out using a new ball and a used disc

covered with ca. 45 nm of ZDDP tribofilm. In order to

prepare this used disc, a mixed sliding–rolling test was

Fig. 10 Intensity results of elements using SEM–EDX

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Fig. 11 Mapping results of elements as obtained using SEM–EDX

Fig. 12 Film thicknesses as obtained using AFM

46 Page 8 of 11 Tribol Lett (2016) 64:46

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carried out under the following conditions: 20 N load,

50 % slide roll ratio (125 mm/s in disc speed and 75 mm/s

in ball speed), 130 �C and 45 min. The used disc was then

left in place, but the used ball was replaced by a new ball,

the used lubricant changed for a fresh sample, and a uni-

directional pure sliding test was then carried out at 10 N

load, 160 mm/s disc speed and 130 �C. The succession of

SLIM images obtained is shown in Fig. 16. At the begin-

ning of the test, a build-up of tribofilm was observed in left

side of the contact area on the ball. Since, in the SLIM

images shown, lubricant enters the contact area from left

side and leaves from the right side, this suggests that tri-

bofilm from the used disc is transferred into the inlet of the

contact area on the ball. From Fig. 16, it can be seen that

Fig. 13 Mapping results of elements as obtained using SEM–EDX under unidirectional pure sliding condition

Fig. 14 Initial film thicknesses under unidirectional pure sliding condition

Fig. 15 Intensity results of elements using SEM–EDX on the disc

Tribol Lett (2016) 64:46 Page 9 of 11 46

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there was no significant wear of the ball during the test,

unlike what was observed when a fresh ball is used as in

Fig. 4.

This means that when a disc already covered by Zn and

P tribofilm is used, no significant damage on the ball occurs

even in unidirectional pure sliding condition. Therefore, Zn

and P tribofilms can show good anti-wear performance in

unidirectional pure sliding conditions only if enough tri-

bofilm is built up before initial wear takes place. Thus, in

unidirectional sliding wear takes place initially because the

ZDDP film does not form fast enough on the ball. This

suggests that initial S tribofilm has important role for

protecting the steel surface from initial wear because Zn

and P tribofilm formation takes a longer time to be built up

than S tribofilm.

4.9 Hypothesis of How Tribofilms Build Up

on the Ball in Pure Sliding Reciprocating

Conditions

From this study, the following hypothesis of how tribofilms

build up on the ball in pure sliding reciprocating conditions

is suggested (Fig. 17). Firstly, high sulphur-content tri-

bofilm material is built up in certain areas. Secondly, the

size of the high sulphur-content tribofilm particles increa-

ses. Thirdly, these tribofilm particles break up into smaller

fragments and move to the front and rear to form two

distinct regions. Finally, Zn and P tribofilms are created,

mainly between the two regions and on top of the initial

sulphur particles. If insufficient tribofilm forms on the

moving countersurface, possibly because this spends too

high a proportion of the test time out of contact, then the

high sulphur contest still forms on the ball, but the second,

Zn/P tribofilm formation on the ball is very slow and

unable to prevent wear.

5 Conclusion

The authors have developed a new approach to study

ZDDP tribofilm formation in pure sliding reciprocating

conditions based on the MTM–SLIM method. Key con-

clusions are as follows.

1. In the unidirectional pure sliding condition, the worn

area on the ball expands with test time, indicating

considerable wear. By contrast to this, in the recipro-

cating pure sliding conditions, tribofilm forms on the

ball and no significant damage occurs. In addition,

there is no significant influence of load, disc frequency

and stroke length on the tribofilm forming process and

no surface damage occurs on the balls under any

conditions tested.

2. In the initial tribofilm formation on the ball under

reciprocating pure sliding conditions, solid particulate

tribofilms with a high concentration of S form initially

in the centre of the contact area. Subsequently, the

tribofilm separates into two regions near the front and

rear of the contact, before reforming a pasty tribofilm

between these regions with a high concentration of Zn

and P.

3. In order to avoid any damage to the ball surface, it is

essential to build up a ZDDP tribofilm on the disc;

once tribofilm forms on the disc surface, no further

significant damage on the ball surface occurs even if a

new ball is employed.

In most previous research on ZDDP tribofilm formation,

attention has focussed on the thick glassy Fe phosphate and

Zn phosphate layers after rubbing test. This study has

shown that in pure sliding reciprocating conditions, a sul-

phur-rich thicker tribofilm is initially formed and removed

from the steel surface, and then, the phosphate-based layer

develops. Such behaviour may suggest that sulphur content

providing extreme pressure response has important role for

protecting the steel surface from initial wear even though

Fig. 16 Series of SLIM images

of the ball in unidirectional pure

sliding using a new ball and a

disc covered by Zn and P

tribofilms

Fig. 17 Hypothesis of how tribofilms build up on the ball in pure

sliding reciprocating conditions

46 Page 10 of 11 Tribol Lett (2016) 64:46

123

zinc and phosphorus tribofilms subsequently build up to

provide prolonged anti-wear protection.

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